1. Field of the Invention
The present invention relates to a method of disinfecting fresh vegetables by processing the same with a liquid containing a mixture of argon:carbon dioxide, and also a method of maintaining carbon dioxide concentrations in aqueous solutions at a given level by adding argon to the system.
2. Description of the Background
It is known that nitrogen may be used to displace oxygen in order to prevent oxidation or to limit respiration. Carbon dioxide may be used for the same purpose and is also used in beverages to provide effervescence.
It is further known that carbon dioxide may be used as a microbicidal or microbiostatic agent or as a solute in disinfecting solutions.
Vegetable products have been subjected to chlorinating treatments for disinfection. Unfortunately, treatment with chlorine imparts an off-taste and causes considerable chemical damage to the final product. This damage is manifested by bleached color, poor taste, poor aroma retention, oxidation of product, poor texture, poor flavor and fragrance profiles and an overall quality which is perceived as poor by the customer.
Although it is known that carbon dioxide can be dissolved in aqueous solution to produce a carbonated disinfecting solution, such a solution is not as effective as a chlorinated washing solution. Further, oxidative deterioration of vegetables is observed with products washed with carbonated solutions as well as with chlorinated solutions.
Generally, several methodologies are presently known for disinfecting foods.
For example, U.S. Pat. No. 4,454,723 describes a refrigerated trailer cooled by sprinkler water with concomitant release of nitrogen from a cryogenic source, for inerting the respiration of produce.
U.S. Pat. No. 4,522,835 shows that gases containing molecular oxygen in their molecules are often reactive with food systems. Examples of such gases are O.sub.2, CO.sub.2, and CO. Preservation of color in meat, poultry, fish may be obtained by reducing O.sub.2 content to produce myoglobin/hemoglobin versus the ordinary oxidized states of oxymyoglobin/hemoglobin, and then adding CO to product carboxymyoglobin/carboxyhemoglobin. Thereinafter, the produce is stored under CO.sub.2 to maintain the improved color. Storage under inert N.sub.2 is possible, as is further reordination using O.sub.2.
EP 354337 describes the use of CO.sub.2 as an antibacterial agent in the preservation of foods, and the effectiveness of this method has been widely demonstrated.
U.S. Pat. No. 3,535,124 describes a process for preservation of fresh fruit juice in a dispenser using CO.sub.2, wherein the juice is preferably sprayed through an inert atmosphere.
U.S. Pat. No. 3,587,203 describes the gas packaging of tossed salad, which is cut and prepared ready to eat, where it is stored in an inert atmosphere in order to prevent oxidative discoloration.
Zee et al., 1984, describe the effects of CO, CO.sub.2, and N.sub.2 on bacterial growth on meat under gas packaging. Argon was used as an inert control. It was found that Ar and N.sub.2 were equivalent in inhibiting growth of anaerobes, acting as inerting agents therefor. Specifically, four strict aerobes, three anaerobes, and twelve facultative anaerobes isolated from meat were grown under CO.sub.2, Ar, N.sub.2, CO, where Ar was found to be "inert" even when containing 10-70% N.sub.2, CO.sub.2, or CO. However, the effect of Ar was found to be strictly to the gases with which it was admixed.
A two-step treatment process for the treatment of fresh fruits and vegetables is described in FR 2652719, wherein nitrous oxide (10-100%) in admixture with O.sub.2 and/or CO.sub.2 is applied to vegetables in a first phase of treatment, followed by a separate second phase treatment with a gas mixture which contains nitrous oxide (N.sub.2 O) (10-99%) admixed with O.sub.2 or CO.sub.2 or N.sub.2, whereby preservation is effected by action of N.sub.2 O. Therein, it is clearly described that nitrogen or argon are equivalent inert gases which may be freely used to complement in bulk any given gas mixture without effect.
N.sub.2 O is shown to prevent ethylene formation and provides significant fungistatic activity. Nitrous oxide is widely used in food packaging and preservation are known, e.g., FR 2191850.
FR Pat. Appl. 31/7/1990 and EP 0422995 also describe a two-step treatment for preserving fresh vegetables by exposure at refrigerating temperatures under an atmosphere of nitrous oxide and/or argon (other noble gases are specifically claimed to be insert) and optimally oxygen. Mixtures used variously include high titers of N.sub.2 O, O.sub.2, CO.sub.2 or N.sub.2.
In essence, the disclosed two-step process, which is not simple gas packaging, entails applying N.sub.2 O or argon which directly interfere with the production of ethylene by tomatoes. Argon is described as having a specific utility in this regard, however, it is clear from the data presented therein that argon only functions to displace oxygen from the tissues of the fruit and thereby to limit respiration, and thereby ethylene production. It is also clear that that the data presented show a difference in ethylene production of air, N.sub.2, Ar, and N.sub.2 O which is precisely identical to their differences in solubility in the fruit. The present inventors have established as much by duplicating the above experiment wherein adequate controls for solubility were made by including other gases. Thereby, it has been ascertained that depression of ethylene may be completely explained by the displacement of oxygen.
The above experiments with food demonstrate only that argon is inert, and, moreover, confirm that preservation with argon is superior to simple air displacement with "inert" gases. Further, effects are only noted when the product is packaged in the gas of interest, or when significant retention of gas is manifested in the package. No transient treatment combining argon and carbon dioxide is either described or suggested.
Addition of argon to a mixture of CO.sub.2 +O.sub.2 is described in U.S. Pat. No. 4,946,326, as being effective in preserving seafood and fish at 4.degree. C., packaged under the mixture. The mixture contains, in partial pressure, 5-68% CO.sub.2, 5-20% O.sub.2, and 27-45% Ar, preferably 50:20:30/CO.sub.2 :O.sub.2 :Ar. This patent describes the mixture as slowing down enzymatic and chemical reactions at the surface of and inside of fish and seafood products, as well as growth of some microorganisms, such as fungus.
The results described therein, however cannot be considered reproducible, and are due entirely to careful control of processing hygiene, and the effects of CO.sub.2 on microbes. The data presented are not statistically significant, and, moreover, do not control for the obvious effects of CO.sub.2, O.sub.2, and N.sub.2 alone or in mixtures without noble gases. Clearly, it cannot be concluded, therefore, that any observed effect is, in fact, due to argon or to any specific mixture disclosed therein.
EP 354337 describes the effect of CO.sub.2 upon bacterial systems. Such effects are widely known and are understood as being caused by the depressive effect of CO.sub.2 upon ordinary respiratory processes, which cannot be interpreted as being specifically anti-enzymatic. The results described in U.S. Pat. No. 4,946,326 can be largely attributed to the simple depression of respiration by CO.sub.2.
Thus, a need exists for a means by which fresh vegetables may be washed without attendant bleaching and general oxidative degradation as is caused by carbon dioxide.
At the same time, very little is known regarding methods by which control of carbon dioxide concentrations in aqueous solution may be effected. Although nitrogen has been used to strip carbon dioxide from liquids, the present inventors are aware of no prior art which describes a more effective means for controlling carbon dioxide concentrations in solution.
Thus, a need also exists for a more effective means of controlling carbon dioxide concentrations in an aqueous solutions containing the same.